Eric S. Raymond's former Design Notes On Fetchmail

These notes are for the benefit of future hackers and
maintainers. The following sections are both functional and
narrative, read from beginning to end.

History

A direct ancestor of the fetchmail program was originally
authored (under the name popclient) by Carl Harris
<ceharris@mal.com>. I took over development in June 1996 and
subsequently renamed the program `fetchmail' to reflect the
addition of IMAP support and SMTP delivery. In early November 1996
Carl officially ended support for the last popclient versions.

Before accepting responsibility for the popclient sources from
Carl, I had investigated and used and tinkered with every other
UNIX remote-mail forwarder I could find, including fetchpop1.9,
PopTart-0.9.3, get-mail, gwpop, pimp-1.0, pop-perl5-1.2, popc,
popmail-1.6 and upop. My major goal was to get a header-rewrite
feature like fetchmail's working so I wouldn't have reply problems
anymore.

Despite having done a good bit of work on fetchpop1.9, when I
found popclient I quickly concluded that it offered the solidest
base for future development. I was convinced of this primarily by
the presence of multiple-protocol support. The competition didn't
do POP2/RPOP/APOP, and I was already having vague thoughts of maybe
adding IMAP. (This would advance two other goals: learn IMAP and
get comfortable writing TCP/IP client software.)

Until popclient 3.05 I was simply following out the implications
of Carl's basic design. He already had daemon.c in the
distribution, and I wanted daemon mode almost as badly as I wanted
the header rewrite feature. The other things I added were bug fixes
or minor extensions.

After 3.1, when I put in SMTP-forwarding support (more about
this below) the nature of the project changed -- it became a
carefully-thought-out attempt to render obsolete every other
program in its class. The name change quickly followed.

The rewrite option

MTAs ought to canonicalize the addresses of outgoing non-local
mail so that From:, To:, Cc:, Bcc: and other address headers
contain only fully qualified domain names. Failure to do so can
break the reply function on many mailers. (Sendmail has an option
to do this.)

This problem only becomes obvious when a reply is generated on a
machine different from where the message was delivered. The two
machines will have different local username spaces, potentially
leading to misrouted mail.

Most MTAs (and sendmail in particular) do not canonicalize
address headers in this way (violating RFC 1123). Fetchmail
therefore has to do it. This is the first feature I added to the
ancestral popclient.

Reorganization

The second thing I did reorganize and simplify popclient a lot.
Carl Harris's implementation was very sound, but exhibited a kind
of unnecessary complexity common to many C programmers. He treated
the code as central and the data structures as support for the
code. As a result, the code was beautiful but the data structure
design ad-hoc and rather ugly (at least to this old LISP
hacker).

I was able to improve matters significantly by reorganizing most
of the program around the `query' data structure and eliminating a
bunch of global context. This especially simplified the main
sequence in fetchmail.c and was critical in enabling the daemon
mode changes.

IMAP support and the method table

The next step was IMAP support. I initially wrote the IMAP code
as a generic query driver and a method table. The idea was to have
all the protocol-independent setup logic and flow of control in the
driver, and the protocol-specific stuff in the method table.

Once this worked, I rewrote the POP3 code to use the same
organization. The POP2 code kept its own driver for a couple more
releases, until I found sources of a POP2 server to test against
(the breed seems to be nearly extinct).

The purpose of this reorganization, of course, is to trivialize
the development of support for future protocols as much as
possible. All mail-retrieval protocols have to have pretty similar
logical design by the nature of the task. By abstracting out that
common logic and its interface to the rest of the program, both the
common and protocol-specific parts become easier to understand.

Furthermore, many kinds of new features can instantly be
supported across all protocols by modifying the one driver
module.

Implications of smtp forwarding

The direction of the project changed radically when Harry
Hochheiser sent me his scratch code for forwarding fetched mail to
the SMTP port. I realized almost immediately that a reliable
implementation of this feature would make all the other delivery
modes obsolete.

Why mess with all the complexity of configuring an MDA or
setting up lock-and-append on a mailbox when port 25 is guaranteed
to be there on any platform with TCP/IP support in the first place?
Especially when this means retrieved mail is guaranteed to look
like normal sender- initiated SMTP mail, which is really what we
want anyway.

Clearly, the right thing to do was (1) hack SMTP forwarding
support into the generic driver, (2) make it the default mode, and
(3) eventually throw out all the other delivery modes.

I hesitated over step 3 for some time, fearing to upset
long-time popclient users dependent on the alternate delivery
mechanisms. In theory, they could immediately switch to .forward
files or their non-sendmail equivalents to get the same effects. In
practice the transition might have been messy.

But when I did it (see the NEWS note on the great options
massacre) the benefits proved huge. The cruftiest parts of the
driver code vanished. Configuration got radically simpler -- no
more grovelling around for the system MDA and user's mailbox, no
more worries about whether the underlying OS supports file
locking.

Also, the only way to lose mail vanished. If you specified
localfolder and the disk got full, your mail got lost. This can't
happen with SMTP forwarding because your SMTP listener won't return
OK unless the message can be spooled or processed.

Also, performance improved (though not so you'd notice it in a
single run). Another not insignificant benefit of this change was
that the manual page got a lot simpler.

Later, I had to bring --mda back in order to allow handling of
some obscure situations involving dynamic SLIP. But I found a much
simpler way to do it.

The moral? Don't hesitate to throw away superannuated features
when you can do it without loss of effectiveness. I tanked a couple
I'd added myself and have no regrets at all. As Saint-Exupery said,
"Perfection [in design] is achieved not when there is nothing more
to add, but rather when there is nothing more to take away." This
program isn't perfect, but it's trying.

The most-requested features that I will never add, and why
not:

Password encryption in .fetchmailrc

The reason there's no facility to store passwords encrypted in
the .fetchmailrc file is because this doesn't actually add
protection.

Anyone who's acquired the 0600 permissions needed to read your
.fetchmailrc file will be able to run fetchmail as you anyway --
and if it's your password they're after, they'd be able to rip the
necessary decoder out of the fetchmail code itself to get it.

All .fetchmailrc encryption would do is give a false sense of
security to people who don't think very hard.

Truly concurrent queries to multiple hosts

Occasionally I get a request for this on "efficiency" grounds.
These people aren't thinking either. True concurrency would do
nothing to lessen fetchmail's total IP volume. The best it could
possibly do is change the usage profile to shorten the duration of
the active part of a poll cycle at the cost of increasing its
demand on IP volume per unit time.

If one could thread the protocol code so that fetchmail didn't
block on waiting for a protocol response, but rather switched to
trying to process another host query, one might get an efficiency
gain (close to constant loading at the single-host level).

Fortunately, I've only seldom seen a server that incurred
significant wait time on an individual response. I judge the gain
from this not worth the hideous complexity increase it would
require in the code.

Multiple concurrent instances of fetchmail

Fetchmail locking is on a per-invoking-user because
finer-grained locks would be really hard to implement in a portable
way. The problem is that you don't want two fetchmails querying the
same site for the same remote user at the same time.

To handle this optimally, multiple fetchmails would have to
associate a system-wide semaphore with each active pair of a remote
user and host canonical address. A fetchmail would have to block
until getting this semaphore at the start of a query, and release
it at the end of a query.

This would be way too complicated to do just for an "it might be
nice" feature. Instead, you can run a single root fetchmail polling
for multiple users in either single-drop or multidrop mode.

The fundamental problem here is how an instance of fetchmail
polling host foo can assert that it's doing so in a way visible to
all other fetchmails. System V semaphores would be ideal for this
purpose, but they're not portable.

I've thought about this a lot and roughed up several designs.
All are complicated and fragile, with a bunch of the standard
problems (what happens if a fetchmail aborts before clearing its
semaphore, and how do we recover reliably?).

I'm just not satisfied that there's enough functional gain here
to pay for the large increase in complexity that adding these
semaphores would entail.

Multidrop and alias handling

I decided to add the multidrop support partly because some users
were clamoring for it, but mostly because I thought it would shake
bugs out of the single-drop code by forcing me to deal with
addressing in full generality. And so it proved.

There are two important aspects of the features for handling
multiple-drop aliases and mailing lists which future hackers should
be careful to preserve.

The logic path for single-recipient mailboxes doesn't involve
header parsing or DNS lookups at all. This is important -- it means
the code for the most common case can be much simpler and more
robust.

The multidrop handing does not rely on doing the
equivalent of passing the message to sendmail -t. Instead, it
explicitly mines members of a specified set of local usernames out
of the header.

We do not attempt delivery to multidrop mailboxes in
the presence of DNS errors. Before each multidrop poll we probe DNS
to see if we have a nameserver handy. If not, the poll is skipped.
If DNS crashes during a poll, the error return from the next
nameserver lookup aborts message delivery and ends the poll. The
daemon mode will then quietly spin until DNS comes up again, at
which point it will resume delivering mail.

When I designed this support, I was terrified of doing anything
that could conceivably cause a mail loop (you should be too).
That's why the code as written can only append local names
(never @-addresses) to the recipients list.

The code in mxget.c is nasty, no two ways about it. But it's
utterly necessary, there are a lot of MX pointers out there. It
really ought to be a (documented!) entry point in the bind
library.

DNS error handling

Fetchmail's behavior on DNS errors is to suppress forwarding and
deletion of the individual message that each occurs in, leaving it
queued on the server for retrieval on a subsequent poll. The
assumption is that DNS errors are transient, due to temporary
server outages.

Unfortunately this means that if a DNS error is permanent a
message can be perpetually stuck in the server mailbox. We've had a
couple bug reports of this kind due to subtle RFC822 parsing errors
in the fetchmail code that resulted in impossible things getting
passed to the DNS lookup routines.

Alternative ways to handle the problem: ignore DNS errors
(treating them as a non-match on the mailserver domain), or forward
messages with errors to fetchmail's invoking user in addition to
any other recipients. These would fit an assumption that DNS lookup
errors are likely to be permanent problems associated with an
address.

IPv6 and IPSEC

The IPv6 support patches are really more protocol-family
independence patches. Because of this, in most places, "ports"
(numbers) have been replaced with "services" (strings, that may be
digits). This allows us to run with certain protocols that use
strings as "service names" where we in the IP world think of port
numbers. Someday we'll plumb strings all over and then, if inet6 is
not enabled, do a getservbyname() down in SocketOpen. The IPv6
support patches use getaddrinfo(), which is a POSIX p1003.1g
mandated function. So, in the not too distant future, we'll zap the
ifdefs and just let autoconf check for getaddrinfo. IPv6 support
comes pretty much automatically once you have protocol family
independence.

Internationalization

Internationalization is handled using GNU gettext (see the file
ABOUT_NLS in the source distribution). This places some minor
constraints on the code.

Strings that must be subject to translation should be wrapped
with GT_() or N_() -- the former in function arguments, the latter
in static initializers and other non-function-argument
contexts.

Checklist for Adding Options

Adding a control option is not complicated in principle, but
there are a lot of fiddly details in the process. You'll need to do
the following minimum steps.

Add a field to represent the control in struct
run, struct query, or struct
hostdata.

Go to rcfile_y.y. Add the token to the grammar.
Don't forget the %token declaration.

Pick an actual string to declare the option in the .fetchmailrc
file. Add the token to rcfile_l.

Pick a long-form option name, and a one-letter short option if
any are left. Go to options.c. Pick a new
LA_ value. Hack the longoptions table to
set up the association. Hack the big switch statement to set the
option. Hack the `?' message to describe it.

If the default is nonzero, set it in def_opts near
the top of load_params in
fetchmail.c.

Add code to dump the option value in
fetchmail.c:dump_params.

For a per-site or per-user option, add proper
FLAG_MERGE actions in fetchmail.c's optmerge()
function. For a global option, add an override at the end of
load_params; this will involve copying a "cmd_run." field to a
corresponding "run." field, see the existing code for models.

Document the option in fetchmail.man. This will require at
least two changes; one to the collected table of options, and one
full text description of the option.

Hack fetchmailconf to configure it. Bump the fetchmailconf
version.

Hack conf.c to dump the option so we won't have a version-skew
problem.

Add an entry to NEWS.

If the option implements a new feature, add a note to the
feature list.

There may be other things you have to do in the way of logic, of
course.

Before you implement an option, though, think hard. Is there any
way to make fetchmail automatically detect the circumstances under
which it should change its behavior? If so, don't write an option.
Just do the check!

Lessons learned

1. Server-side state is essential

The person(s) responsible for removing LAST from POP3 deserve to
suffer. Without it, a client has no way to know which messages in a
box have been read by other means, such as an MUA running on the
server.

The POP3 UID feature described in RFC1725 to replace LAST is
insufficient. The only problem it solves is tracking which messages
have been read by this client -- and even that requires
tricky, fragile implementation.

The underlying lesson is that maintaining accessible server-side
`seen' state bits associated with Status headers is indispensible
in a Unix/RFC822 mail server protocol. IMAP gets this right.

2. Readable text protocol transactions are a Good Thing

A nice thing about the general class of text-based protocols
that SMTP, POP2, POP3, and IMAP belongs to is that client/server
transactions are easy to watch and transaction code correspondingly
easy to debug. Given a decent layer of socket utility functions
(which Carl provided) it's easy to write protocol engines and not
hard to show that they're working correctly.

This is an advantage not to be despised! Because of it, this
project has been interesting and fun -- no serious or persistent
bugs, no long hours spent looking for subtle pathologies.

3. IMAP is a Good Thing.

Now that there is a standard IMAP equivalent of the POP3 APOP
validation in CRAM-MD5, POP3 is completely obsolete.

4. SMTP is the Right Thing

In retrospect it seems clear that this program (and others like
it) should have been designed to forward via SMTP from the
beginning. This lesson may be applicable to other Unix programs
that now call the local MDA/MTA as a program.

5. Syntactic noise can be your friend

The optional `noise' keywords in the rc file syntax started out
as a late-night experiment. The English-like syntax they allow is
considerably more readable than the traditional terse keyword-value
pairs you get when you strip them all out. I think there may be a
wider lesson here.

Motivation and validation

It is truly written: the best hacks start out as personal
solutions to the author's everyday problems, and spread because the
problem turns out to be typical for a large class of users. So it
was with Carl Harris and the ancestral popclient, and so with me
and fetchmail.

It's gratifying that fetchmail has become so popular. Until just
before 1.9 I was designing strictly to my own taste. The multi-drop
mailbox support and the new --limit option were the first features
to go in that I didn't need myself.

By 1.9, four months after I started hacking on popclient and a
month after the first fetchmail release, there were literally a
hundred people on the fetchmail-friends contact list. That's pretty
powerful motivation. And they were a good crowd, too, sending fixes
and intelligent bug reports in volume. A user population like that
is a gift from the gods, and this is my expression of
gratitude.

The beta testers didn't know it at the time, but they were also
the subjects of a sociological experiment. The results are
described in my paper, The
Cathedral And The Bazaar.

Credits

Special thanks go to Carl Harris, who built a good solid code
base and then tolerated me hacking it out of recognition. And to
Harry Hochheiser, who gave me the idea of the SMTP-forwarding
delivery mode.